1. Field of the Invention
This invention relates to a semiconductor memory device. For example, this invention relates to a semiconductor memory device including a MOS transistor having a floating gate and a control gate.
2. Description of the Related Art
With the recent rapid advances in the field of large-scale integrated circuits (LSIs), the affect of plasma damage caused in manufacturing processes on semiconductor elements is becoming more serious. This problem is particularly serious in LSIs using a large number of multilayer wiring lines, because many contact opening processes are carried out.
To overcome this problem, a method of using a protective diode in a one-chip microcomputer having a logic circuit, a semiconductor memory, an MCU, and others embedded to a single chip to cause charges contributive to plasma damage to escape to the semiconductor substrate has been disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 2000-332202.
In recent years, a flash memory combining the features of the NOR flash memory and the NAND flash memory has been proposed in, for example, Wei-Hua Liu, “A 2-Transistor Source-select (2TS) Flash EEPROM for 1.8-V-Only Application,” Non-Volatile Semiconductor Memory Workshop 4.1, 1997. A flash memory of this type has memory cells, each including two MOS transistors. In such a memory cell, one MOS transistor functioning as a nonvolatile memory section has a structure including a control gate and a floating gate and is connected to a bit line. The other MOS transistor, which is connected to a source line, is used to select a memory cell.
As described above, in the conventional LSI, an approach of using a protective diode to avoid plasma damage has been proposed. In the case of LSI with a flash memory including memory cells, each having two MOS transistors as described in the reference, sufficient operating reliability has not been achieved by the conventional method of avoiding plasma damage, because of the use of a positive voltage and a negative voltage in operation and the necessity of guaranteeing that the memory cells can be rewritten more than a hundred thousand times.